Buried structures, particularly metallic structures such as a gas pipeline, undergo a corrosion process due to the galvanic activity at the interface between the surface of the pipe and the surrounding soil. If the potential is measured between the pipe and a second buried electrode, a naturally occurring battery will be observed with the soil forming the electrolyte.
A copper electrode having a copper sulfate surface is widely adopted as a standard reference electrode for measuring the potential between a buried electrode and a pipeline for determining the level of pipeline corrosion protection. A natural, freely corroding steel surface will typically show a free corrosion potential between such a standard reference electrode and the pipe in the range -0.62 volts to -0.68 volts.
A common method of retarding the corrosion is to impress a current through the soil to the pipe from an external, rectified alternating current source which opposes the current of the corrosion process. This makes the pipe the cathode of the resulting galvanic cell and therefore prevents or at least mitigates corrosion on the buried pipeline. During cathodic protection, the impressed current increases the cathodic, or reduction reaction on the pipeline surface while simultaneously decreasing the corrosion or oxidation reaction. Cathodic protection is applied by an impressed current cathodic protection system which typically uses a rectifier to apply an average DC cathodic current to a pipeline by connecting the pipeline to the negative terminal of the rectifier output and an inert buried anode to the positive terminal.
Criteria have been established to measure the effectiveness of the impressed current cathodic protection system. One important criterion is the measurement of the polarized potential of the pipeline using the standard copper/copper sulfate reference electrode. The measured polarized potential is the difference in the potentials of the pipe and the reference electrode in the soil electrolyte while in their protected state. This is the most commonly used criterion and provides that corrosion is assumed sufficiently mitigated when the polarized potential between a steel pipe and the standard reference electrode is -0.85 volts. That is, when the polarized potential at the pipe is -0.85 volts with respect to the standard reference electrode, the pipe is considered properly protected.
A variety of problems exist, however, with attempts to measure this polarized potential. First, as recognized in U.S. Pat. No. 4,591,792, the impressed protection system within the soil. Thus, the polarized potential, while the protection current is impressed, cannot be directly measured. Instead, while the current is impressed, the potential measured between the reference electrode and the pipe is the algebraic sum of the polarized potential and the IR drop in the soil between the pipe and the reference electrode.
One solution to this IR drop problem, which has been commonly used, is to interrupt the impressed protection current and measure the potential between the pipe and the reference electrode during the interruption interval. The theory is that such interruption will eliminate the IR drop in the soil and thus permit direct measurement of the polarized potential.
One problem with such interruption systems arises from the fact that a pipeline is ordinarily protected by a series of discrete protection circuits spaced along the length of the pipeline. Therefore, the potential measured at a test site between the pipeline and a reference electrode includes the sum of the pipe to reference electrode IR drops contributed by each of the protection circuits. Thus, for the interruption system to work, the interruption must be synchronized so that all interruptions occur simultaneously.
Although synchronization is expensive, it is currently used on some protection systems to facilitate the measurement of the polarized potential. However, further problems exist which are not addressed by such synchronization systems. A long pipeline behaves electrically as an inductor. That inductive property, together with other reactive circuit elements, create exponentially decaying transients or spikes when the current of such systems is switched. In particular, immediately after the impressed protection current is switched off, and again immediately after it is switched back on, a transient spike is created which requires a finite interval of time to decay. Thus, the interruption of the impressed protection current does not result in an immediate drop of the potential between the pipe and the standard electrode to the polarized potential. Instead, the potential drops further by an amount of an inductive spike resulting from the collapse of the magnetic field about the pipe as its energy is returned into the circuit. The result is that the potential between the pipe and the standard electrode, immediately after interruption, is not the polarized potential, but rather is the sum of the polarized potential and this inductive spike. Only after the inductive spike has decayed will the potential between the pipe and the standard electrode reach the polarized potential.
Still another problem with interruption systems is that after the protection current is interrupted and after the transient decays, the pipe interface begins to depolarize as the system begins to electrochemically revert back to its natural battery operation. Thus, the potential measured between the reference electrode and the pipe begins to return from the polarized potential at which the pipeline is protected to its free corrosion potential. Therefore, after a substantial time has elapsed, the potential between the pipe and the standard electrode will no longer be the polarized potential.
This latter problem is aggravated by the fact that currently used metering devices, such as a strip chart or a galvanometer-type meter, have a substantial, mechanical inertia associated with them. As a result, they require a substantial period of time until they can react to a change in potential. The response time of such devices is ordinarily sufficiently long that some reversion of the potential toward the free corrosion potential has occurred before a steady state reading is obtained. In fact, the response time of strip charts and galvanometers is so slow that they will not indicate the inductive spike and will settle to a steady state only after some reversion of the potential between the pipe and the standard electrode from the polarized potential toward the free corrosion potential. It thus becomes a matter of art and interpretation and consequent inaccuracy to determine from these devices the actual polarized potential.
Finally, yet another problem which exists in all of these systems is the problem of the presence of noise or unwanted signals in the form of alternating currents in the soil even after the interruption of protection circuits. These noise currents are in the form of 50 Hz or 60 Hz currents which have been picked up from electrical equipment in the region and some RF noise.
Therefore, it is an object and feature of the present invention to eliminate any need for synchronizing the switching off of multiple spaced protection circuits distributed along the pipeline, to eliminate the problems which arise from such extraneous alternating current noise signals and to obtain a reliable measurement of the polarized potential E.sub.OFF after the reactive spike has decayed but before the potential makes any substantial reversion from the polarized potential which exists during operation of the protection system toward the free corrosion potential.